1. Field of the Invention
The present invention relates to a communications apparatus which performs a cipher communication, and more particularly, to a communications apparatus such as a facsimile apparatus having a cipher communication function.
2. Description of the Prior Art
In the facsimile communication, various special communications can be performed as options in communication between facsimile apparatuses of the same manufacturer. The cipher communication is one of such special communications.
Referring to FIG. 4, there is shown the procedure of a typical cipher communication. A signal processing procedure on the transmitting side is shown at 91 to 93. The image data is encoded at 91. Then, to make the communication secret, the encoded image data is enciphered by using a key at 92 and transmitted at 93. A signal processing procedure on the receiving side is shown at 94 to 96. The cipher data received at 94 is deciphered at 95. The decipherment is to clear the cipher by using the same key as that used on the transmitting side. The deciphered data is encoded at 96 and returned to the original image data. The key is a kind of data.
In the image communication with a facsimile apparatus, there is always a possibility that a data error occurs according to the condition of the telephone line. If the data on the transmitting side is not correctly sent to the receiving side due to the data error, from the time of occurrence of the data error, a data error occurs on the receiving side in deciphering/decoding the data, so that the data sent from the transmitting side cannot be returned to the original state. As a result, a correct image cannot be obtained.
For example, a response signal transmitted from the receiving side every time the transmission of data for one page or a part of a page is completed serves as a response signal representative of a re-transmission request (PPR) when the data are not correctly received on the receiving side. In this case, of the data for one page or a part of a page stored in the memory, the transmitting side re-transmits the data requested by the receiving side.
Referring to FIG. 5, there is shown an example of a data of an HDLC format used for an ECM communication. In data of the HDLC format, one frame includes encoded signals of a flag F, an address A, a control C, an image data FIF and a check CRC arranged in this order. Each frame is of a predetermined size.
On the receiving side, when a signal such as a procedure completion signal (PPS-EOP), a message completion signal (PPS-EOM), a multi-page signal (PPS-MPS) and a partial page signal (PPS-NULL) (these signals will be referred to as PPS-Q en bloc) is received from the transmitting side, if there is an error in a received frame, the frame is specified, and a partial page request signal (PPR) is transmitted to request the re-transmission of the data.
On the transmitting side, the transmitted data are stored in the memory in case of errors, and the frame including an error is re-transmitted in response to the request of the receiving side. The maximum number of frames transmitted in response to the partial page request signal is 256, and the maximum bytes of the data transmitted in one frame is 256. Therefore, on the transmitting side, a memory of at least 256 frames.times.256 bytes=64 kilobytes is necessary for the ECM communication.
However, according to the ECM communication, since it is necessary to store the data in the memory of the transmitting side as described above, the ECM communication cannot be performed when the cipher communication is performed and when it is necessary to transmit a large amount of data.
Specifically, the cipher communication is specified in the beginning of transmission, and the control unit performs an operation for the cipher communication. In executing the cipher communication function, a large memory capacity is necessary compared to the normal communication. For this reason, when the cipher communication is performed, it may be impossible to secure the memory capacity necessary for the ECM communication.
If the memory capacity necessary for the ECM communication cannot be secured when the cipher communication is performed, a data error may occur on the receiving side, so that when the re-transmission of the data is requested, the transmitting side cannot re-transmit the data. When the re-transmission cannot be performed, the ECM communication is not effected at that point of time, so that it is impossible to surely transmit the data to the receiving side.